[113.07] The Systemic Proper Motions of the Magellanic Clouds and their Orbits around the Milky Way

The interaction between the Large and Small Magellanic
Clouds (LMC & SMC) and the Milky Way (MW) is thought to
have played an important role in the dynamical evolution of
the MW's outer parts. The Clouds probe the potential of the
MW dark halo in places where other kinematic tracers are
unavailable and thus the MW-MC system has been a major
subject of study. In particular, the global dynamics of both
Clouds need to be well prescribed before other evolutionary
features of the system can be understood. The radial
velocities of the clouds are more readily determined than
the transverse velocities, which can only be estimated using
proper motions.

We undertook a project using two epochs of HST/ACS data to
determine the systemic proper motions of the Clouds. The
Magellanic Cloud fields are centered on background QSOs that
were discovered from their optical variability in the MACHO
database (Geha et al. 2003). The final sample consists of 21
QSOs behind the LMC and 5 behind the SMC, distributed
homogeneously behind the central few degrees of both Clouds.
With a ~ year baseline and the use of the High
Resolution Camera, we have determined the proper motion of
the LMC to better than 5 \ \muN = 0.44 ±0.05 mas/yr
(Kallivayalil et al. 2005). This is the most accurate proper
motion measurement for any MW satellite thus far. We will
present this measurement, as well as our results for the
SMC, and the conclusions we can draw about the Clouds'
orbits around the MW.

Our study shows that ground-based work on finding QSOs can
be combined with high resolution HST data to get good
measurements in a relatively short amount of time. When
combined with HI data from the Magellanic Stream our
measurements should provide new constraints on both the mass
distribution of the Galactic Halo and models of the Stream.

Support for this work was provided by NASA through grant
numbers GO-09462 and GO-10130 from STScI. KHC's work was
performed under the auspices of the U.S. DOE, NNSA, by the
Univ. of California, LLNL under contract No. W-7405-Eng-48.